The present invention generally relates to a data storage and reduction method for digital images having a chronological order. More particularly, the present invention relates to a surveillance system using such a method and having a camera, a digital storage device and an image storage controller operatively connected to the camera and the storage device for receiving a plurality of images from the camera and storing the images digitally in the storage device, the images having a chronological order reflecting the points of time, at which the images were produced. The invention also relates to a method of storing such digital images.
Surveillance camera systems are commonly used for monitoring various areas in place of business, such as cashier windows at a bank or gambling tables at a casino. Traditionally, such surveillance systems have included analog video cameras, which continuously record all activities within a surveillance zone on a magnetic tape cassette, such as a standard VHS cassette. Typically, such cassettes have a nominal recording time of a few hours, e.g. 2, 4 or 8 hours. When the tape cassette has been fully recorded, the cassette will have to be substituted by an empty one by a human operator.
The maximum recording time may be increased by reducing the speed, at which the magnetic tape is moved in relation to the recording head, thereby reducing the number of frames stored per second. In this way the maximum recording time for a standard VHS tape may be extended to e.g. 48 hours, thereby obviously also extending the time between cassette substitution correspondingly. However, the penalty for not having to switch tapes as frequently is a substantial reduction in image quality for the entire recording time.
In recent years, digital video cameras have entered the market. Such cameras produce a sequence of digital snapshot images or still images at a constant rate. For instance, European-type cameras produce images at a rate of 25 images or frames per second (fps), while US-type cameras operate at 30 fps. When the sequence of images is played back, the rapid switch between images is transparent to a human observer. Instead, the sequence of snapshot images is experienced as a continuous stream of moving images.
The output of digital cameras is stored on an appropriate storage medium, such as a magnetic disk (hard disk), an optical disk (CD, DVD) or a digital magnetic tape (DAT). Similarly, the output of analog cameras may be digitised into a sequence of digital images to be stored on such a storage medium.
The images are usually compressed by way of a data compression algorithm, such as JPEG, MPEG, wavelet or fractal compression, wherein certain image information is eliminated and the amount of storage space is reduced accordingly. Since each digital image anyway requires a substantial amount of storage space (typically 10 to 1000 KB), even if a high-density, state-or-the-art digital storage medium is employed, all available storage space will eventually be consumed by the sequence of images produced by the camera. In such a situation, one option is to manually switch to another digital storage medium with free storage space, for instance by having an operator exchange a magnetic disk, an optical disk or a digital tape cassette.
However, being dependent of a human operator for regularly providing additional storage space is undesired in many applications, particularly in applications where the surveillance system is to be used during non-office hours, for instance during evenings or nights.
An obvious way of automatically handling situations, where the currently used storage medium have become completely or almost completely filled with digital images, is to sacrifice the oldest images on a first-in-first-out (FIFO) basis, so that the storage space occupied by such old images are continuously reused for storing new images.
However, this approach has at least two drawbacks, the first of which being the limited preservation of historic image information. Every storage medium has a maximum amount of storage space available for storing digital information. Irrespective of the actual value of the maximum amount of storage space, each storage medium will only be able to store a certain number of images, this number being dependent of the maximum storage capacity of the storage medium, the pixel resolution of each image, and the frequency or rate (fps) of these images. For instance, if a particular storage medium is only capable of storing a sequence of images corresponding to e.g. two hours of recording time, then all events that have occurred more than two hours ago will be completely and irrevocably lost, when such an automatic procedure for reusing storage space is employed.
A second drawback is that such an automatic FIFO procedure inherently operates under the assumption that the oldest image information is always the least important. However, in many situations certain older image information may actually be of higher importance than certain younger image information, for instance if an alarm situation has occurred and has been incorporated in such older image information.
It is an object of the present invention to provide a substantial improvement to camera-based surveillance systems. In particular, an object of the invention is to chronologically extend the scope of image information recorded and stored by a surveillance camera system, so that chronologically old image information may be kept for a longer period of time than with the prior art systems.
A further object is to provide enhanced representation of images related to a historic alarm event. Yet another object is to provide an option for a user of the system to dynamically define the chronological image information coverage provided by the system.
Generally, the objects above are achieved by storing new digital images at a high image quality and provide time-dependent reduction in image quality for certain older images or groups of images, thereby reducing the storage space required for such certain images.
More particularly, the main objects of the invention are achieved by a data storage and reduction method for digital images having a chronological order, wherein a first and chronologically new image or group of images is/are stored at a first image quality requiring a first amount of storage space, and wherein a second image or group of images, chronologically older than said first image or group of images, is processed by reducing the image quality thereof to a second image quality requiring a second amount of storage space, which is less than said first amount of storage space.
The main objects of the present invention are also achieved for a surveillance system comprising a camera, a digital storage device and an image storage controller operatively connected to the camera and the storage device for receiving a plurality of images from the camera and storing the images digitally in the storage device, the images having a chronological order reflecting the points of time, at which the images were produced, by providing the image storage controller with: first means for determining a qualitative value for individual images, or groups of images, stored in the storage device, the qualitative value being a function of a chronological position of the respective image or group of images; second means for selecting, among the images stored in the storage device, a particular image or group of images in response to its qualitative value; and third means for compressing or deleting said particular image or group of images.
Other objects, advantages and features of the present invention will be apparent when studying the following detailed disclosure, the appended claims as well as the attached drawings.